The ultimate purpose of the Ashtanga practice is purification of the body and mind. By maintaining each pose with ease and power we can eliminate all physical and mental blocks.
Breath is life! We enter this world riding upon the wings of an inhale and depart while exhaling. Life is truly one breath. Breath awareness is the central hub of the yoga wheel around which all else rotates. In Ashtanga we create a sound when we breathe. This sound acts as a mantra or metre upon which we meditate and focus while practicing Yoga.
The most important and in fact the only description of asana given in the Yoga Sutras is “sthira sukham asanam”, meaning that every asana should be 'STEADY' and 'COMFORTABLE' - STHIRA and SUKHA. 'Sthira' means steady or stable or grounded or strong and 'Sukha' means comfortable or easy (or 'easeful') or peaceful.